There are two common forms of canker, fungal canker and bacterial canker.
The fungul type is a common and destructive diseases of apple trees, and may also affect pears, beech, ash, poplar, and sorbus.
If left unattended small branches can be girdled and killed in a single season.
The fungus spreads from the edge of the canker during the dormant season and produces spores, these spores are then spread by wind and rain into wounds which were possibly made by woolly aphids.
What happens is: an area of bark dies, and gradually forms a hollow, and the fungus forms at its edges, sometimes resulting in an infection so bad that it can kill whole branches.
In summer, white pustules appear on the sunken bark, these pustules are filled with spores, which when disturbed are spread by wind, these develop into small red bodied spores which remain on the tree throughout the winter months.
Small branches or spurs infected by canker should be cut out and burnt.
On larger branches and main stems, the brown diseased areas should be cut out with a sharp knife until a clean white or green-white wound is left.
Paint the wound with white-lead paint or grafting wax, or proprietary canker paint.
Destroy infected material!
In severe attacks, spray the trees with Bordeaux mixture or liquid copper, after the fruit is picked, but before leaf-fall,and again when half the leaves have fallen, then finally the following spring at bud-swelling stage.
Heavy, waterlogged soil may aggravate canker.